The present invention relates generally to the field of optics. More particularly, the invention provides a method and system for predicting and correcting optical distortions. Merely by way of example, the system has been applied in a laser system, but it would be recognized that the invention has a much broader range of applicability. It would be recognized that the invention can be applied to other optical applications that may have disturbances from thermal influences.
Laser systems have been widely used for sending high energy electromagnetic radiation as a beam of a single wavelength from a source to a target. The laser beam carries signals, energy, or both. Laser systems have been employed in applications such as medical, industrial, and defense. Medical applications include selectively removing diseased tumors from human bodies. Industrial applications include metal fabrication and other ways of ablating materials. Defense applications have also used laser systems.
As merely an example, such defense applications include use of a high-energy chemical oxygen iodine laser (COIL) carried aboard a large aircraft such as a Boeing 747-400F freighter. The aircraft carrying the high energy laser (called herein “Airborne Laser”) is capable of autonomous operation at altitudes above the clouds. The Airborne Laser is designed to locate and track objects such as missiles in certain phases of their flight. The Airborne Laser accurately points and fires the high-energy laser to the missiles, which destroy the missiles near their launch areas. Other descriptions of the Airborn Laser can be found at http://www.airbornelaser.com.
Although the Airborne Laser has been highly successful, it is desirable to further improve the Airborne Laser or other laser systems. When the laser beam travels from the source to the target, the laser beam could become distorted by certain disturbances. Such disturbance can come from various sources, such as a change of atmospheric index of refraction over the optical path of the beam. This is commonly called optical turbulence. Additionally, disturbances often come from the laser systems themselves, such as distortions in the optical systems due to heating of the optical components by the passage of the laser transmission. Turbulences must be compensated for, and also local optical distortions in order to improve the quality of the laser beam transmission.
To compensate for such disturbances and turbulences, certain characteristics of these must be determined. Using the Airborne Laser System as an example, the laser guidestar technology could be used. Two laser guidestar technologies are the Rayleigh beacon method and the mesosphere beacon method. In both methods, a laser system focuses a beacon laser beam in the outgoing direction of the beacon laser beam. The Rayleigh beacon method usually focuses the beacon laser beam at about 15 to 25 kilometers away from the laser system; while the mesosphere beacon method focuses the laser beam at the mesosphere, which is about 92 kilometers away from the sea level. In either method, the light returning from the focus of the beacon laser beam serves as a wavefront source for sampling turbulences along the path between the focus and the laser transmitter such as the Airborne Laser Aircraft. The sampled turbulences distort the wavefront of backscattered signals. This distortion is usually analyzed by a wavefront sensor and then reconstructed by computer. Then the conjugate of the reconstructed distortion is placed on the outgoing laser beam. By this careful pre-distortion of the transmitted beam, the distortion along the path just refocuses the beam on the target.
FIG. 1 illustrates a simplified process for estimating distortion and correcting laser beam. This figure is merely an illustration. Aircraft 110 flies over clouds 120. Clouds 120 usually top at about 38,500 feet above sea level, and aircraft 110 usually flies at about 40,000 to 45,000 feet above the sea level. Aircraft 110 is equipped with laser system 130. The laser system 130 sends a track laser beam 140 to lock onto nose 150 of missile 160. Subsequently, laser system 130 sends a beacon laser beam 170 to area 175 of missile 160 for estimating distortion of laser beam due to various turbulences. Using this return beacon information, the pre-distorted high energy laser beam 180 is sent out along the same path. During the round trip time the missile rises up and the high energy laser beam 180 hits the “sweet spot” desired 190. As noted above and further emphasized here, FIG. 1 is merely an illustration. High energy laser beam 180 is usually emitted after beacon laser beam 170. During the interval, sweet spot 190 may have traveled to the vicinity of the original location of area 175; hence high energy laser beam 180 may take roughly the same orientation as beacon laser beam 170.
FIG. 2 is a simplified diagram for the Airborne Laser system 130. Laser system 130 usually includes nose-mounted turret 210, beam control system 220, and COIL system 230. COIL system 230 provides high energy laser beam 180 for laser system 130. Internal heating of the optics by laser system 130 may distort laser beams 140, 170, and 180 due to aberrations produced within the system. These aberrations may occur to various components of the system, including conformal window of nose-mounted turret 210 and its boundary layer, the optical system of nose-mounted turret 210, and other optical components along the optical path of the laser beams. To improve performance of the laser beams, the wavefront distortions need to be corrected.
Hence it is desirable to improve wavefront compensation techniques.